Say I'm behind some NAT (the type doesn't matter), and I want to send packets to the public Internet. The first time I send a packet, my packet gets NATed and the NAT allocates and maps some external port number for my packet. If I continue (very frequently) to send packets with the same source port from my private machine, the same NAT mapping persists as far as I can tell (by examining packet addresses on the server destination).

However, if I wait ~1-2 minutes between sending a packet, the next time I send a packet, the NAT appears to have recycled the NAT mapping and assigns a different external port. How long do NATs typically wait until they recycle a mapping? Is there any minimum time that all NATs conform to (like a standard spec)?

1 Answer
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The closest thing to an answer I could find was page 14 of draft-audet-nat-behave-00. This document states NATs must not expire dynamic UDP bindings before 2 minutes with a recommended time of 5 minutes, and 7800 seconds for TCP bindings.

In practice, we have ended up sending a trivial refresh packet every 50 seconds to ensure our UDP binding remains active in our application.